Add a new UMA enum to record how many download uses parallel requests

content/browser/download/parallel_download_job.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_
 #define CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_
 
 #include <memory>
 #include <unordered_map>
 #include <vector>
@@ skipping 16 matching lines @@
       DownloadItemImpl* download_item,
       std::unique_ptr<DownloadRequestHandleInterface> request_handle,
       const DownloadCreateInfo& create_info);
   ~ParallelDownloadJob() override;
 
   // DownloadJobImpl implementation.
   void Start() override;
   void Cancel(bool user_cancel) override;
   void Pause() override;
   void Resume(bool resume_request) override;
+  bool UsesParallelRequests() const override;
 
  protected:
   // Virtual for testing.
   virtual int GetParallelRequestCount() const;
 
  private:
   friend class ParallelDownloadJobTest;
 
   using WorkerMap =
       std::unordered_map<int64_t, std::unique_ptr<DownloadWorker>>;
@@ skipping 36 matching lines @@
 
   // If we have sent parallel requests.
   bool requests_sent_;
 
   DISALLOW_COPY_AND_ASSIGN(ParallelDownloadJob);
 };
 
 }  //  namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_